Recently, an organic electroluminescence (EL) display using electroluminescence (EL) of an organic material has been attracting attention as one of next-generation flat panel displays to replace the liquid crystal display. In an active-matrix display apparatus such as an organic EL display, a thin-film semiconductor device referred to as a thin film transistor (TFT) is used.
Particularly, the organic EL display, unlike the liquid crystal display that is voltage-driven, is a current-driven device, so that it is urgently expected to develop a thin-film transistor having an advantage in ON-OFF characteristics as a driving circuit used for the active-matrix display apparatus. In the configuration of the thin-film transistor, a gate electrode, a semiconductor layer (channel layer), a source electrode, and a drain electrode are formed above the substrate, and it is common to use a silicon thin film for the channel layer.
In addition, for the display device, a larger screen and lower costs are expected, and as a thin-film transistor that can readily achieve lower costs, a bottom-gate thin-film transistor in which the gate electrode is formed closer to the substrate than to the channel layer is generally used.
The bottom-gate thin-film transistor is largely divided into two types: a channel-etching thin-film transistor whose channel layer is etched, and a thin-film transistor of a channel protection type (etching stopper type) which protects the channel layer from the etching processing.
The channel-etching thin-film transistor has an advantage of allowing reduction in the number of photolithography processes compared to the thin-film transistor of the channel protection type, thus suppressing production costs.
On the other hand, the thin-film transistor of the channel protection type can prevent damage to the channel layer caused by etching processing, thus suppressing unevenness in characteristics within a substrate surface. In addition, the thin-film transistor of the channel protection type has an advantage because the channel protection type allows further thinning the channel layer to reduce a parasitic resistance component, thus increasing ON characteristics.
For this reason, the thin-film transistor of the channel protection type is appropriate for, for example, a driving transistor in the organic EL display apparatus using a current-driven organic EL device, and it is attempted to adopt such a channel-protection thin-film transistor for the driving circuit of the organic EL display apparatus even if the manufacturing cost increases compared to the channel-etching TFT.
For example, Patent Literature 1 discloses a channel-protection TFT including a microcrystalline semiconductor thin film as a channel layer, and describes forming a channel protection film above a channel layer via a buffer layer.